Flue gas recirculation has been used in various types of systems to reduce noxious exhaust emissions. A prior art flue gas recirculation apparatus, used in a boiler system, is illustrated in FIG. 1. A boiler system 100 includes a boiler 110 that has a burner, represented at 115 with a controlled air input 116. An exhaust stack, represented at 120, exhausts the flue gases from the system via breeching 117. A recirculation section 130 includes a damper 132 that communicates with the exhaust stack 120 and permits a portion of the flue gases to be drawn by a blower 134 through a duct 135 that couples back to a further input of the burner 115. A sensor 136, which senses noxious emissions (typically, oxides of nitrogen, or "NO.sub.x ") is coupled to a controller 137, the output of which controls a recirculation supply damper 138.
In operation, a selected fraction of the flue gases is recirculated to the burner. The prior art literature indicates that flue gas recirculation acts as a flame quencher, reducing combustion temperatures by thermal dilution. In doing so, among other indicated advantages, it significantly reduces excess air requirements, flame temperature, and flue gas heat loss, thereby reducing NO.sub.x emissions and improving boiler efficiency (see e.g. G . Tompkins, "Flue Gas Recirculation Works For Packaged Boilers Too", POWER, April, 1990). In the FIG. 1 apparatus, the sensor 136 and controller 137 sense the concentration of noxious emissions in the exhaust gas, increase the recirculation fraction when NO.sub.x emissions increase, and reduce the recirculation fraction when NO.sub.x emissions decrease.
Although existing flue gas recirculation techniques are useful in reducing noxious emissions and improving boiler efficiency, they have limitations. For example, although noxious emissions tends to decrease as the fraction of recirculated flue gas is increased, there is a limit on the fraction of recirculated flue gas that can be fed back to the burner input. The upper limit is approximately 25% recirculation. Above this level, the burner flame tends to become unstable, which can severely limit the efficiency of the burner. Accordingly, further reductions in noxious emissions that might result from higher percentage flue gas recirculation generally cannot be achieved.
It is among the objects of the present invention to attain further reduction in noxious emissions without undue sacrifice of flame stability and/or burner efficiency.